The thermotherapy (hyperthermia) method is attracting attention as a method for cancer therapy. By this thermotherapy method, attention is focused upon the fact that a cancer cell or a cancerous tissue is more vulnerable to heat than a healthy cell. By heating the cancer-affected portion for example to 43 degree C. for a certain length of time, the cancer lesion can selectedly be necrotized. In this thermotherapy method, an aqueous sol of magnetic fluid such as a complex of dextran or its derivative and magnetic iron oxide is injected into the affected portion, to which a strong magnetic field is applied from outside to selectively heating the cancer lesion (see Japanese Patent Laid-open Publication No. 2002-360712).
In order to inductively heat the magnetic material having been injected into the affected portion of the patient, it is necessary to irradiate a strong magnetic flux from outside to the affected portion by using a magnetic field generating device. As a preferred magnetic field generating device for this inductive heating is used a device comprising a series resonant circuit of a coil as a magnetic field generating means and a resonant capacitor being connected to the secondary side of the transformer, in which a large current of high frequency is applied to the coil to generate a magnetic field. Since the intensity of the magnetic field generated by the coil decreases progressively with distance, it is preferable to place the source of the magnetic field as close as possible to the affected portion. For this reason, a bread-shaped coil is used as a coil for generating a magnetic field, whose current path is formed into a spiral shape in a same plane.
As an another preceding example is disclosed a device for heating the cancer-affected portion by irradiating a high-density magnetic flux, in which along the periphery of a solenoid coil are provided dual coils, so that magnetic flux density is enhanced with the outer coil (the reinforcement coil) (see Japanese Patent Laid-open Publication No. 2003-205040). Further, a technique for eliminating a skin effect in a bread-shaped spiral coil comprising entwisted conductive wires is disclosed, in which a conductive wire in the shape of a right-hand spiral and a conductive wire in the shape of a left-hand spiral are doubly arranged on the periphery thereof as coils (see Japanese Patent Laid-open Publication No. 2003-115368). In addition, as a technique for cooling a coil comprising a circular cylinder of an insulation material and a litz wire folded about the periphery of the circular cylinder is disclosed, in which a cold airstream is blown into the circular cylinder (see Japanese Patent Laid-open Publication No. 1997-168494). Also, a technique of providing ferrite at the bottom of a coil to shield an electromagnetic wave is disclosed (see Japanese Patent Laid-open Publication No. 1997-245949).